ZrO2 - containing glass-ceramic

ABSTRACT

A ZrO2-containing glass ceramic is described, the main crystalline phase of which is formed by ZrO2 and which has at least one further crystalline phase, and which can be used in particular in dentistry.

The invention relates to a ZrO₂ -containing glass-ceramic which issuitable in particular for use in dentistry because of its excellentmechanical properties and its processability at low temperatures.

ZrO₂ -containing glass-ceramics are known. ZrO₂ is used in lowconcentrations of up to 5% by wt. in traditional glass-ceramics asnucleating agents. Glass-ceramics containing up to 15% by wt. ZrO₂ arealso described in the prior art. However, they contain no phosphoruspentoxide and no or only very small quantities of lithium oxide.

Thus, described for example in U.S. Pat. No. 4,687,749 areglass-ceramics with enstatite as the main crystalline phase which canhave a ZrO₂ content of up to 15% by wt. The glass-ceramics are free fromphosphorus oxide and can contain only small quantities of lithium oxide,namely up to 2% by wt. Further, the materials are very difficult tomelt, with the result that high temperatures of more than 1200° C. andin particular of approximately 1500° C. are required to process them.

Known from DE-OS 42 07 180 are ZrO₂ -containing glass-ceramics which aresuitable for preparing tooth crowns, but have no phosphorus oxide orlithium oxide whatsoever. The ZrO₂ content is limited to a maximum of15% by wt., since it is otherwise difficult to obtain a homogeneousglass.

In addition to ZrO₂ -containing glass-ceramics, which contain ZrO₂homogeneously dissolved in vitreous state in the starting glass and inwhich ZrO₂ crystals subsequently crystallize through thermal treatmentof the starting glass, sinter products or sintered bodies with ZrO₂ arealso known. In the case of these sintered products, crystalline ZrO₂ inpowder form is mixed to a glass powder and products are obtained by asintering reaction.

Thus, known for example from DE-PS 39 05 895 are cordierite sinteredarticles having up to 50% by wt. ZrO₂ but which do not contain anyphosphorus oxide or lithium oxide whatsoever. The ZrO₂ is not obtainedby controlled crystallization of an appropriate starting glass but addedin powder form. By sintering the ZrO₂ powder with powdered startingglass the desired cordierite-ZrO₂ sintered products are produced. It isa disadvantage of the described products that they cannot be furtherprocessed below 1200° C. by the viscous flow process which isadvantageous in dentistry.

It is therefore the object of the invention to provide glass-ceramicswith a high ZrO₂ content which possess a very good mechanical strength,can be processed at low temperatures of less than 1200° C., can bemoulded in particular by pressing, and form a very good adhesive bondwith sintered ZrO₂ ceramics and accordingly can be used in advantageousmanner as materials for moulded dental products.

This object is achieved by the ZrO₂ -containing glass-ceramic accordingto the present invention. The invention also relates to a process forthe preparation of the glass-ceramic, to its use, and to molded dentalproducts which contain the glass-ceramic.

The ZrO₂ -containing glass-ceramic according to the invention comprisesthe following components:

    ______________________________________                                               Component     % by wt.                                                 ______________________________________                                               SiO.sub.2     42.5 to 58.5                                                    Al.sub.2 O.sub.3                                                                            0 to 7.0                                                        La.sub.2 O.sub.3                                                                            0 to 9.5                                                        Li.sub.2 O    7.0 to 14.5                                                     Na.sub.2 O    0 to 7.5                                                        K.sub.2 O     0 to 13.5                                                       P.sub.2 O.sub.5                                                                             4.0 to 13.5                                                     ZrO.sub.2     15.0 to 28.0                                                    TiO.sub.2     0 to 6.0                                                        F             0 to 2.0                                                        BaO           0 to 6.5                                                        CaO           0 to 6.0                                                        B.sub.2 O.sub.3                                                                             0 to 3.3                                                        CeO.sub.2     0 to 3.0                                                 ______________________________________                                    

and further comprises a ZrO₂ crystalline phase and at least one othercrystalline phase. The glass-ceramic preferably consists essentially ofthe given components.

There are particularly preferred quantity ranges for some of thecomponents, and these can be chosen independently of one another and areas follows:

    ______________________________________                                        SiO.sub.2   47 to 55% by wt.                                                  Al.sub.2 O.sub.3                                                                          0 to 5% by wt.                                                    Li.sub.2 O  7 to 14% by wt.                                                   Na.sub.2 O  0 to 6% by wt.                                                    K.sub.2 O   0 to 8% by wt.                                                    P.sub.2 O.sub.5                                                                           5 to 11% by wt.                                                   ZrO.sub.2   16 to 25% by wt.                                                  F           0 to 1.5% by wt.                                                  ______________________________________                                    

The procedure for preparing the glass-ceramic according to the inventionis that a starting glass which contains the required components isfirstly prepared in conventional manner. For this, suitable startingmaterials, e.g. oxides, oxyhydroxides, carbonates and/or phosphates, areusually melted at temperatures of 1550° to 1600° C. until a homogeneousglass melt is obtained.

The prepared starting glass is then subjected to a heat treatment, as aresult of which a controlled crystallization is brought about andconsequently the glass-ceramic is formed. There are severalpossibilities for carrying out the heat treatment and any possiblepreceding moulding step.

The molten starting glass can for example be firstly cast to give adesired moulded article which is then subjected to the heat treatment byheating to a temperature in the range from 580°-1100° C. for 0.5 to 2hours.

It is also possible that a desired moulded article is formed (a) eitherby casting molten starting glass or (b) by uniaxial or isostaticcold-pressing and subsequent sintering of granulated starting glass, andcompressing this in the viscous state at a temperature of 850°-1200° C.The crystallization of the starting glass is effected.

(1) by the sintering of the starting glass,

(2) by the hot-pressing at 850°-1200° C. and/or

(3) by a further heat treatment, carried out if necessary after thehot-pressing.

The possibility given here of the heat treatment by hot-pressingillustrates at the same time the particular advantageousness of theglass-ceramic according to the invention compared with traditionalhigh-strength glass-ceramic materials which cannot be compressed at suchlow temperatures to give moulded articles. Compressing in the viscousstate is preferably carried out using the process and pressing ovendescribed in EP-A-0 231 773.

In a preferred embodiment, a further heat treatment can, as alreadymentioned, be carried out in the temperature range from 580°-1100° C.after the heat treatment by pressing in the viscous state.

It has been shown that when the proportion of ZrO₂ in the starting glassis increased to more than 28% by wt. a homogeneous glass melt can nolonger be obtained. The solubility limit for the glass system usedaccording to the invention obviously lies in this range. For example,when 33% by wt. ZrO₂ was used, undissolved ZrO₂ was still present aftermelting. In such a case the glass-ceramics according to the invention,which are obtained by controlled devitrification of a homogeneousstarting glass with formation of ZrO₂ crystals distributed in highlydisperse manner, cannot be achieved.

The glass-ceramics according to the invention were analyzed in moredetail by means of scanning electron microscope investigations. It wasshown that these are distinguished by a characteristic structure ofdifferently sized crystals. The crystals typically have an average sizeof 20 μm relative to the number of crystals. However, crystal sizes ofless than or equal to 5 μm, in particular less than or equal to 1 μm,are preferred. It is possible that there are still present between thesecrystals finely crystalline particles less than approximately 400 nm insize which almost touch one another or are in direct contact with oneanother.

From the results of X-ray diffraction tests and material contrastinvestigations in the scanning electron microscope, it is concluded thatthe larger crystals are a SiO₂ modification, in particular cristobalite,and/or lithium phosphate. The smaller crystals are ZrO₂ crystals, inparticular as baddeleyite and/or in tetragonal form, which preferablyform the main crystalline phase, and lithium silicate crystals (Li₂SiO₃). In some cases, aluminum phosphate (AlPO₄), lithium aluminumsilicate and, in a low concentration, even lithium zirconium silicate(Li₂ ZrSi₆ O₁₅) are also present.

The ZrO₂ crystalline phase preferably contains ZrO₂ in tetragonal form.The tetragonal modification of the ZrO₂ crystals is preferred, becausethrough it a change to the monoclinic modification can take place uponexposure to an external force, e.g. crack growth. The modificationchange leads to the known effect of inhibition of crack propagation andtherefore to the increase in strength and fracture toughness.

The structure outlined above is typical for the glass-ceramics accordingto the invention, irrespective of whether they are present as cast orcompressed glass-ceramics. Differences only arise as regards the volumeproportion of the individual crystalline phases and the size of thecrystallites in the individual phases. It is assumed that the individualcrystalline phases lead to the increase in the strength of the glassceramics according to the invention as a result of adispersion-strengthening effect. The way in which this exactly takesplace is still unclear at the present time. In this connection it isalso to be considered surprising that the incorporation of cristobalitebrings about no negative influence on the strength despite the knowndifferences in the expansion coefficients of its modifications.

Because of their special chemical composition and their specialstructure the glass-ceramics according to the invention have severaladvantages which make them particularly suitable as dental materials.Firstly, they have very high flexural strengths of up to 400 MPa. Theyalso show a good temperature change resistance and can be obtainedeither with a high degree of whiteness, as a result of the high contentof ZrO₂ crystals, or in translucent form, which is of particularsignificance for dental materials and the moulded dental productsprepared from them. Colouring the glass-ceramics according to theinvention is however also possible. This can be carried out a) bycolouring the starting glass by adding oxides of the 3-d elements and/orof the 4-f elements and/or by metal colloids or b) by adding colourpigments to the starting glass granulate. It is likewise possible to addfluorescent agents. The glass-ceramics can also contain other usualadditives, provided they do not hinder crystallization of the startingglass.

The glass-ceramics according to the invention can also be processed attemperatures below 1200° C., for which purpose in particular thehot-pressing process in the viscous state, which is advantageous for thepreparation of dental products, is used. A moulding of conventionalhigh-strength glass-ceramic materials is frequently not possible atthese low temperatures. It is also a particular advantage that, incontrast to conventional glass ceramics, the glass-ceramics according tothe invention do not react either with the investment material, which isused in the preparation of moulded dental products employinghot-pressing processes. This is an essential advantage for the dentaltechnician processing them.

Finally, the glass-ceramics according to the invention adhere very wellto the high-strength pure ZrO₂ ceramics, which is important particularlyfor use in dentistry. Thus, for example, a suitable ZrO₂ glass-ceramiccan be pressed against a high-strength ZrO₂ ceramic post directlyfollowing individual moulding, i.e. depending on the cavity in question.The ZrO₂ ceramic post is thus anchored firmly in the tooth and furthertooth construction can be undertaken.

In view of the properties explained above, the glass-ceramic accordingto the invention is also preferably used as (a) a dental material ordental product moulded therefrom or as (b) a constituent of a dentalmaterial or of a dental product moulded therefrom. Preferred dentalproducts are tooth root structures, in particular posts and cores.

The invention is explained in more detail below with reference toexamples.

EXAMPLES Examples 1 to 18

A total of 18 different glass-ceramics according to the invention wereprepared. They had the chemical compositions given in Table I.

In addition to indications of the preparation process selected in eachcase, details of various properties are given in Table II for some ofthese glass-ceramics. The values given for the flexural strength areaverage values from several measurements according to ISO 6872-1984, andthe measured values deviate by a maximum of ±30 to 40 MPa from theseaverage values. It is pointed out that in many cases, in addition to thecrystalline phases given in the table, yet further crystalline phasesformed in a sometimes low concentration and size, but it was not howeverpossible to identify them unequivocally by radiographic means.

Nos. 1 and 6 are examples of cast glass-ceramics and these were preparedaccording to the process described in Example 19, the heat treatmentused in each case being indicated in Table II.

Examples of compressed glass-ceramics are nos. 3, 7, 10 and 11, andthese were obtained according to the process described in Example 20.Glass-ceramics nos. 3, 7, 10 and 11 were prepared according to variant A(casting, fine cooling, compressing in the viscous state) and glassceramic no. 3 also according to variant B (fritting, cold-pressing,sintering, compressing in the viscous state). If after compressing inthe viscous state, a further heat treatment was carried out, then thisis indicated in the table as "thermal post-treatment".

The examples illustrate how glass-ceramics with different structures andproperties can be obtained by changing the chemical composition of thestarting glass and the preparation process.

                                      TABLE I                                     __________________________________________________________________________    Compositions of glass ceramics according to the invention: (Data in % by      wt.)                                                                          1      2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17 18                     __________________________________________________________________________    SiO.sub.2                                                                         52.8                                                                             44.2                                                                             50.8                                                                             45.2                                                                             42.6                                                                             51.0                                                                             52.2                                                                             51.5                                                                             53.6                                                                             46.3                                                                             49.4                                                                             50.7                                                                             58.2                                                                             52.2                                                                             47.0                                                                             49.7                                                                             52.0                                                                             47.8                   Al.sub.2 O.sub.3                                                                  3.0                                                                              3.7                                                                              2.9                                                                              6.7                                                                              3.6                                                                              2.3                                                                              3.0                                                                              3.0                                                                              1.2                                                                              3.2                                                                              2.8                                                                              -- 1.3                                                                              3.0                                                                              2.8                                                                              3.0                                                                              1.2                                                                              2.8                    La.sub.2 O.sub.3                                                                  -- 4.5                                                                              -- 2.3                                                                              4.4                                                                              -- -- -- -- -- -- -- -- -- 9.5                                                                              -- -- --                     Li.sub.2 O                                                                        12.9                                                                             11.1                                                                             8.3                                                                              11.4                                                                             10.8                                                                             12.4                                                                             10.0                                                                             9.9                                                                              13.1                                                                             7.3                                                                              8.9                                                                              9.2                                                                              14.2                                                                             9.2                                                                              11.8                                                                             12.8                                                                             13.2                                                                             8.3                    Na.sub.2 O                                                                        -- -- 2.7                                                                              -- -- -- 2.9                                                                              2.9                                                                              -- -- -- -- -- 7.2                                                                              -- -- -- 3.1                    K.sub.2 O                                                                         -- 2.6                                                                              4.3                                                                              2.7                                                                              2.5                                                                              -- -- -- -- 13.3                                                                             9.7                                                                              10.0                                                                             -- -- -- -- -- 4.0                    CaO -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 6.0                                                                              --                     BaO -- 6.3                                                                              -- -- 6.1                                                                              -- -- -- -- -- -- -- -- -- -- -- -- --                     P.sub.2 O.sub.5                                                                   10.4                                                                             12.5                                                                             10.0                                                                             13.2                                                                             12.5                                                                             8.1                                                                              10.3                                                                             10.2                                                                             4.3                                                                              11.3                                                                             9.7                                                                              10.0                                                                             4.7                                                                              7.8                                                                              9.6                                                                              10.4                                                                             4.3                                                                              10.3                   B.sub.2 O.sub.2                                                                   -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 3.3                                                                              -- --                     ZrO.sub.2                                                                         20.9                                                                             15.1                                                                             20.1                                                                             18.5                                                                             17.5                                                                             20.2                                                                             20.7                                                                             20.5                                                                             27.8                                                                             18.6                                                                             19.5                                                                             20.1                                                                             21.6                                                                             20.6                                                                             19.3                                                                             20.8                                                                             21.3                                                                             19.7                   TiO.sub.2                                                                         -- -- -- -- -- 6.0                                                                              -- -- -- -- -- -- -- -- -- -- 2.0                                                                              --                     F   -- -- 0.9                                                                              -- -- -- 0.9                                                                              2.0                                                                              -- -- -- -- -- -- -- -- -- 1.0                    CeO.sub.2                                                                         -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 3.0                    __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________                                                       3-point                                                Appearance,                                                                           Crystalline phases forming                                                                   flexural                   Example No.                                                                             Specimen preparation and heat treatment                                                         optical behaviour                                                                     in the structure                                                                             strength                   __________________________________________________________________________                                                        MPa!                      Cast glass ceramic                                                                      Bulk glass block: whitish,                                                                              Crystalline phases (<300                                                                     357:                       Starting glass:                                                                         850° C./0.5 h                                                                            translucent                                                                           ZrO.sub.2 : (tetragonal and                                                   baddeleyite)                              cast, bulk                          SiO.sub.2 : (cristobalite)                                                    Lithium phosphate                         1                                                                             Cast glass ceramic                                                                      Bulk glass block: white   ZrO.sub.2 : tetragonal (<200                                                                 404                        Starting glass:                                                                         950° C./1 h        Lithium phosphate (<400 nm)               cast, bulk                          SiO.sub.2 : (cristobalite) < 1.5                                              μm                                     3                                                                             Pressed glass ceramic                                                                   Bulk glass ingot: white   Lithium phosphate < 5                                                                        305.m                      Starting glass:                                                                         Compressed in the viscous state at                                                                      ZrO.sub.2 < 500 nm                        cast, bulk                                                                              1050° C.:                                                              10' holding time/5' pressing time                                   3                                                                             Pressed glass ceramic                                                                   Sintered glass ceramic ingot:                                                                   white   Lithium phosphate < 5                                                                        180.m                      Starting glass:                                                                         Compressed in the viscous state at                                                                      ZrO.sub.2 < 1 μm                       Powder < 90 μm                                                                       1050° C.:          Baddeleyite < 20 μm                              10' holding time/5' pressing time                                   6                                                                             Cast glass ceramic                                                                      Bulk glass block: white   ZrO.sub.2 < 200 nm (tetragonal                                                               277                        Starting glass:                                                                         900° C./1 h        baddeleyite)                              cast, bulk                          SiO.sub.2 (cristobalite) < 1 μm                                            Lithium phosphate < 1 μm               7                                                                             Pressed glass ceramic                                                                   Bulk glass ingot: white   Lithium phosphate < 5                                                                        250.m                      Starting glass:                                                                         Compressed in the viscous state at                                                                      SiO.sub.2 (cristobalite) < 5 μm        cast, bulk                                                                              1050° C.:          ZrO.sub.2 (<200 nm)                                 10' holding time/12' pressing time                                            thermal post-treatment at 1000° C.                           10                                                                            Pressed glass ceramic                                                                   Bulk glass ingot: whitish,                                                                              Lithium phosphate < 500                                                                      199                        Starting glass:                                                                         Compressed in the viscous state at                                                              translucent                                                                           ZrO.sub.2 (tetragonal and                 cast, bulk                                                                              1000° C.:          baddeleyite) < 100 nm                               5' holding time/32' pressing time                                                                       (possibly SiO.sub.2)                                thermal post-treatment at 800° C./3 h                                  and 920° C./5 h                                              11                                                                            Pressed glass ceramic                                                                   Bulk glass ingot: whitish,                                                                              Lithium phosphate < 500                                                                      213                        Starting glass:                                                                         Compressed in the viscous state at                                                              translucent                                                                           ZrO.sub.2 : tetragonal and                cast, bulk                                                                              1000° C.:          baddeleyite < 100 nm                                5' holding time/25' pressing time                                                                       (possibly SiO.sub.2)                                thermal post-treatment at 800° C./3 h                                  and 920° C./5 h                                              __________________________________________________________________________

Example 19 Cast Glass Ceramic

Firstly, a starting glass having the chemical composition given in TableI for glass ceramic no. 1 was prepared. For this, an appropriate mixtureof oxides, oxyhydroxides, carbonates and phosphates was melted in aplatinum/rhodiumcrucible for 2 hours at temperatures of 1500° to 1600°C. The obtained molten glass was cooled in water, i.e. fritted, dried,granulated and melted again for 2 hours at 1500° to 1600° C. in order toachieve a good homogeneity.

The glass melt was then cast to give a bulk glass block weighing approx.100 g, and the glass block was cooled from 650° C. so slowly that nostresses formed in the glass. Rods (approx. 5×2×25 mm) were cut out ofthe glass block and heat-treated at 850° C. for 30 minutes. The flexuralstrength measured for the obtained glass ceramic rods and otherproperties of the glass ceramic and details of its structure are listedin Table II.

The glass ceramic prepared in this Example is suitable in excellentmanner inter alia because of its high strength, translucence and easyprocessability and its white appearance as a dental material, which canbe used e.g. when preparing a tooth root structure.

Example 20 Compressed Glass-Ceramics

Firstly, a starting glass having the chemical composition given in TableI for glass-ceramic No. 3 was prepared. For this, a melt of the startingglass was obtained as in Example 19 by melting twice. The obtained meltwas then further processed according to two different variants (A) and(B).

Variant (A):

Here, the melt of the starting glass was cast to give a bulk glass rod(diameter: 11.3 mm; length: 50 mm) and slowly cooled in order to avoidthe formation of stresses. A small cylindrical solid glass ingot(diameter: 11.3 mm; length: 15 mm) was then cut out of the rod andcompressed in the viscous state using the pressing process and pressingoven according to EP-A-0 231 773 under vacuum and at a temperature of1050° C. and at 5 bar pressing pressure to give the desired specimengeometry.

Compared with conventional materials, it is a particular advantage ofthis glass-ceramic that it can be processed even at temperatures lowerthan 1200° C., namely at 1050° C., to give individually moulded dentalproducts, e.g. can be pressed against high-strength ZrO₂ ceramic posts.Nor does compression lead either to an undesired reaction, whichfrequently occurs with conventional materials, with the requiredinvestment material, which emphasizes the excellent suitability of theglass ceramic according to the invention for the preparation ofindividually moulded dental products.

Variant (B):

Here, the starting glass melt was firstly fritted by pouring it intowater, and the resulting frit was ground and sieved to a grain size ofless than 90 μm. The obtained glass powder was then compressed using auniaxial drying press at 1000 bar pressing pressure to give smallcylinders. The glass cylinders were then sintered in a vacuum at atemperature of 850° C. for 30 minutes in a furnace, as a result of whichdevitrification of the starting glass already took place to a certainextent. The obtained ingots were finally compressed under vacuum in theviscous state using the pressing process and pressing oven according toEP-A-0 231 773 to give the desired specimen geometry.

The glass-ceramic according to the invention obtained in this way alsodisplayed the advantages obtained with variant (A) during processing andwas thus able to be used in advantageous manner e.g. for the preparationof individually moulded restorations for posts.

We claim:
 1. A ZrO₂ -containing glass-ceramic composition, comprising aZrO₂ crystalline phase, a lithium phosphate crystalline phase, and atleast one additional crystalline phase, and comprising the followingcomponents:

    ______________________________________                                               Component     % by wt.                                                 ______________________________________                                               SiO.sub.2     42.5 to 58.5                                                    Al.sub.2 O.sub.3                                                                            0 to 7.0                                                        La.sub.2 O.sub.3                                                                            0 to 9.5                                                        Li.sub.2 O    7.0 to 14.5                                                     Na.sub.2 O    0 to 7.5                                                        K.sub.2 O     0 to 13.5                                                       P.sub.2 O.sub.5                                                                             4.0 to 13.5                                                     ZrO.sub.2     15.0 to 28.0                                                    TiO.sub.2     0 to 6.0                                                        F             0 to 2.0                                                        BaO           0 to 6.5                                                        CaO           0 to 6.0                                                        B.sub.2 O.sub.3                                                                             0 to 3.3                                                        CeO.sub.2     0 to 3.0.                                                ______________________________________                                    


2. The glass-ceramic composition according to claim 1, comprising thefollowing components:

    ______________________________________                                        SiO.sub.2   47 to 55% by wt.                                                  Al.sub.2 O.sub.3                                                                          0 to 5% by wt.                                                    Li.sub.2 O  7 to 14% by wt.                                                   Na.sub.2 O  0 to 6% by wt.                                                    K.sub.2 O   0 to 8% by wt.                                                    P.sub.2 O.sub.5                                                                           5 to 11% by wt.                                                   ZrO.sub.2   16 to 25% by wt.                                                  F           0 to 1.5% by wt.                                                  ______________________________________                                    


3. The glass-ceramic composition according to claim 1, wherein the ZrO₂crystalline phase contains ZrO₂ in tetragonal modification.
 4. Theglass-ceramic composition according to claim 1, wherein a furthercrystalline phase is formed by an SiO₂ crystalline modification and/oran AlPO₄ modification isotypic to SiO₂.
 5. The glass-ceramic compositionaccording to claim 1, wherein the glass-ceramic composition istranslucent.
 6. The glass-ceramic composition according to claim 1,wherein the glass-ceramic composition has a flexural strength of up to400 MPa.